Yup. You took the path down into the chest room. Swim to the room above it and stand on the pathform with the grate on the ground. Clawshot the target above you, then let go to land on the floor below, right next to the chest.

right i guess you can see the boss key in the pillar but can't get to it yeah? right go back to where you blew up the first under water rock / boulder (whatever) and look around! theres another boulder, blow up that one and go through door thats not in the water. bingo. clawshot the target on the celing and lower yourself down